1. Field of the Invention
The present invention relates to a skew control method and a skew control unit which are suitably applied to a recording apparatus or playback apparatus for a disc, such as an optical disc, magnetic optical disc, or phase change disc on/from which data is optically recorded or reproduced (hereinafter, these discs are referred to as merely “optical disc”).
This application claims priority of Japanese Patent Application No. 2003-330248, filed on Sep. 22, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
An optical disc apparatus for recording and reproducing data on/from an optical disc is configured to perform a tracking servo control which allows a laser beam radiated from an optical pick-up to follow a track formed on a surface of the disc to record data on the track, or reproduce the recorded data from the track.
An optical disc like a CD (Compact Disc) has a large capacity and allows random access. Further, since optical reading is performed in a noncontact manner, the optical disc is in less danger of a head crash or wear and damage through reading operations than a contact-type recording medium like a magnetic tape. Moreover, a solid disc surface of the optical disc reduces the possibility of accidental data loss. Such an optical disc, which has many advantages as described above, is a recording medium that serves excellently as a memory used in the peripheral of a personal computer, and is superior in data creation or data save.
In recent years, a recording/playback apparatus using a recordable optical disc called CD-R (Compact Disc-Recordable) has been developed.
Some CD-Rs can easily record data in all standard formats for use in compact discs such as a CD-ROM, a CD-ROM/XA, a CD-I, and a CD-DA. Today, the CD-R has replaced conventional magnetic tape, magnetic disc or the like as a recording/playback apparatus for use in an electronic apparatus for recording or reproducing data, which has increasingly come to be used.
Further, for a DVD, which is an optical disc that handles large capacity data such as video data, a DVD-R (DVD-Recordable) which is of write once type, and a DVD-RW (DVD-Rewritable) and a DVD-RAM (DVD-Random Access Memory) which are rewritable, are available as a recordable optical disc.
With an optical disc, data is read out with a converged light beam focused on a disc surface in general. If the central axis (optical axis) of the converged light beam is not perpendicular to a signal recording surface of the disc, a coma aberration is generated on a focal point surface to degrade characteristics (Modulation Transfer Function etc.) of the read out signal, with the result that data cannot be read out correctly. A DVD or DVD-R has a higher recording density than a CD, and accordingly an NA (Numerical Aperture) of an objective lens is generally increased, so that the coma aberration is increased compared to the tilt between the disc surface and the central axis. That is, when a laser beam is radiated to the track formed on the optical disc, the signal recording surface on which the track is formed must be perpendicular to the radiated laser beam. However, since the optical disc is made of a resin material and the like, there are many cases where a warpage of the disc occurs due to variations in products or moisture absorption, or depending on the state of preservation. Accordingly, when the laser beam scans the track positioned in the portion that the warpage has occurred, the laser beam and the signal recording surface are not perpendicular to each other, with the result that recorded data cannot be read out correctly.
For this reason, skew control has been performed, in which a skew sensor which detects the distance or tilt between a disc surface (or signal recording surface) and an optical pick-up is used to correct a relative tilt between the disc and optical pick-up based on a detection signal of the skew sensor so that a laser beam always strikes the signal recording surface at right angles (refer to, for example, Jpn. Pat. Appln. Laid-Open Publication No. 11-86310).
However, when a dedicated sensor is provided for this skew control, a configuration of the optical pick-up grows correspondingly complex, leading to an increase in production cost and size of the optical pick-up.
Further, when the skew sensor is used for skew control, skew values to be detected change in each case due to aging characteristics or thermal behavior of the sensor, with the result that stable skew control cannot be achieved.